Polyolefins, such as polyethylene (PE), are synthesized by polymerizing monomers, such as ethylene (CH2═CH2). Because it is cheap, safe, stable to most environments and easy to be processed polyolefins are useful in many applications. Polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene) as well as High Molecular Weight (HMW), Medium Molecular Weight (MMW) and Low Molecular Weight (LMW). Each type of polyethylene has different properties and characteristics.
Olefin (such as ethylene) polymerizations are frequently carried out in a loop reactor using monomer (such as ethylene), diluent and catalyst, optionally an activating agent, optionally one or more co-monomer(s), and optionally hydrogen.
Polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles suspended in diluent. The slurry is circulated continuously in the reactor with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solid concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled.
Optionally, the product slurry may be fed to a second loop reactor serially connected to the first loop reactor wherein a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product is a bimodal polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, and has a bimodal molecular weight distribution.
After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried, additives can be added and finally the polymer may be mixed and pelletized.
During the mixing step, polymer product and optional additives are mixed intimately in order to obtain a compound as homogeneous as possible. Preferably, mixing is done in an extruder wherein the ingredients are mixed together and the polymer product and optionally some of the additives are melted so that intimate mixing can occur. The melt is then extruded into a rod, cooled and granulated, e.g. to form pellets. In this form the resulting compound can then be used for the manufacturing of different objects.
It has been found on an industrial scale that while the polymer particles are insoluble or substantially insoluble in the diluent, the polymer product has some tendency to deposit on the walls of the polymerization reactor. This so-called “fouling” can lead to a decrease in the efficiency of heat exchange between the reactor bulk and the coolant around the reactor. This leads in some cases to loss of reactor control due to overheating, or to reactor or down stream polymer processing equipment failure due to formation of agglomerates (ropes, chunks).
This “fouling” is caused in part by fines and also by the build up of electrostatic charge on the walls on the reactor. Attempts to avoid fouling during slurry polymerization have been made by adding an antifouling agent in the polymerization medium. Typically, the antifouling agent acts for example to make the medium more conductive, thus preventing to some extent the formation of electrostatic charge, which is one cause of the build-up of polymer on the wall of the reactor.
However, complications may still occur during polyolefin production such as partial or even complete blockage of the loop reactor. These problems can be even more pronounced with particular polyolefins, such as polyethylenes. Blockage may require stopping the production process to unclog and clean the reactor; only then, production can be resumed.
There remains a need in the art for an improved polyolefin production process, particularly for polyethylene and more particularly for high molecular weight polyethylene of high density, and especially to reduce production costs, control process conditions and/or produce optimal polymer end-products.